1. Field of the Invention
The present invention relates to power supply for a camera, and more particularly, to a power-supply system having a solar battery (or solar cell) and a secondary battery.
2. Description of the Related Art
Lithium batteries with large storage capacities have recently been used as batteries for cameras. Lithium batteries are characterized not only by their large storage capacity but also by a relatively small self-discharge current and low internal-resistance. Lithium batteries may be the optimum power-supply for a camera; however, their drawback is that they still have a high retail price.
Recently, rechargeable lithium secondary-batteries have appeared on the market. Therefore, when the rechargeable secondary battery and a solar battery are used together, the battery power consumed by a camera can automatically be replenished by the solar battery. Thus, a long-lasting power-supply system is achieved.
However, the available surface area of a camera for providing the solar battery thereon is limited. The electromotive force of the solar battery is proportional to its area. Therefore, power obtained by the solar battery provided on the camera is inevitably limited.
In addition, when taking into consideration the fluctuation of the frequency of use of a camera and a condition of sunlight in the place where the camera is used, it is important to definitely inform a camera-user of a ready/not-ready status for photographing by means of clearly displaying the remaining amount of the secondary battery power.
A well-known method of informing a camera-user of the remaining battery power is to apply current from the battery to a load and then to determine the remaining amount of power from the voltage of the battery. In a power-supply system of a camera having a conventional primary battery, the remaining amount of battery power varies only when the camera is being used (except for the case when it varies due to a self-discharging from the battery). Therefore, when the battery-checking operation is conducted right after a releasing operation of the camera and, consequently, the display of the remaining amount of battery power is updated according to the checking results, a sufficiently reliable display of the remaining amount of battery power can be provided.
However, in the system where the solar battery and the secondary battery are used together, the charging operation from the solar battery to the secondary battery is conducted even when the camera is not in use, e.g. being carried or left on a shelf. Therefore, there has been a problem in that the display of the remaining amount of battery power does not provide a reliable indication if the battery-checking operation is not carried out to update the display of the remaining amount of battery power while the secondary battery is being charged.
On the other hand, power is needed for a microcomputer and peripheral analog circuit to carry out the battery-checking operation. Thus, power consumption to some extent in these circuits is unavoidable. Therefore, another undesirable problem occurs when the battery-checking operation is frequently conducted to raise the level of displaying accuracy to cope with an unexpectedly executed charging operation in the camera; namely, increased wasteful power-consumption occurs from the battery-checking operation itself.